The present invention relates to a radio paging receiver and, more particularly, to a radio paging receiver consuming less electric power.
Radio paging receivers in recent radio paging systems usually have such call notifying functions as visual message displaying in addition to alerting by audible tones. In a radio paging receiver having such call notifying functions, first the radio section receives and demodulates a paging signal sent from a base station of a paging system to which the receiver belongs. Then, a pulse shaping circuit shapes the waveform of the demodulated paging signal. The pulse-shaped paging signal is decoded by a decoder, which, if it finds the decoded call number identical with the call number of the receiver stored in a programmable read-only memory (PROM), supplies the decoded message signal to a central processing unit (CPU). The CPU subjects the message signal to such processing as error detection and error correction, and supplies call notifying means with message data resulting from the processing of the message signal. The call notifying means, which notifies the user of a call, has a message display unit for displaying the message data and an audible sound alerting unit for generating alerting sounds, and the CPU gives an instruction to operate either unit or both units of the call notifying means as preset by the user.
For such a radio paging receiver, battery saving, i.e. reducing the consumption of the battery power, is an important requirement. For this reason, a conventional radio paging receiver receives a paging signal not constantly but only intermittently to save the battery power. For instance in a standby (SB) mode which accounts for a predominant part of the total period of use, the radio paging receiver keeps in operation only its decoder, but receives no paging signal. In a receiving (RX) mode which makes a part of the total period of use, the radio section, pulse shaping circuit and decoder are operated to receive and decode a paging signal. In message processing (MP) and message display (MD) modes, in which the receiver is placed only a few times a day, the decoder, CPU and call notifying means are operated, but the power supply to the radio section and pulse shaping circuit is cut off. Thus, the radio paging receiver saves its battery power by keeping in operation only the minimum required circuits.
The radio paging receiver makes available several source voltages to operate its circuits. For instance, one manganese battery, generating a voltage of about 1 volt or more, is used as the power source for the radio section, pulse shaping circuit and audible sound alerting unit, and a secondary power source, whose output is about 5 volts deriving from the manganese battery by a DC--DC converter, is used to supply power to the decoder, CPU and message display unit.
In the above described conventional radio paging receiver, only one DC--DC converter is used as the secondary power source in every operating mode of the receiver. The efficiency of a DC--DC converter, however, varies with the amperage of the load current. For instance, a usual DC--DC converter switches a DC with a transistor and boosts the switched DC with an inductor. However, there is a problem that, where a large-capacity switching transistor is used for the switching purpose, the conversion efficiency of the DC--DC converter drops if the load current is small. Meanwhile there is another problem that, where a small-capacity switching transistor is used, the output voltage of the DC--DC converter drops below the required level if the load current is great.
In an example of radio paging receiver, while the required current of the decoder operating in the SB mode is about 0.2 mA, that of the 5 volts power supply in the MD mode is around 9 mA. Therefore, the DC--DC converter is required to be capable of supplying 9 mA at an output voltage (usually .+-.10% of the rated voltage) within a prescribed range. However, since the power supply efficiency of a DC--DC converter having such a capacity markedly drops at the output current of 0.2 mA, the exhaustion of the battery in the SB mode, which accounts for a major part of the total period of operation, is increased, resulting in a reduced useful life of the battery. As a result, not only does the battery cost of the radio paging receiver increase, but also has the battery to be replaced frequently.